1. Field of the Invention
The invention relates to a switching power supply. More particularly, the invention relates to a switching power supply having a novel magnetic amplifier control circuit.
2. Description of the Related Art
In a conventional switching power supply, an output voltage generated at a secondary side of a transformer in the switching power supply is fed back to a primary side of the transformer in order to stabilize the output voltage of a pulse width-modulated (PWM) controller.
Typically a conventional switching power supply having multiple-outputs, e.g., 5 V, 10 V and so on, uses magnetic amplifiers for control to stabilize each of the output voltages. The magnetic amplifiers are suitable for independently stabilizing the output voltages of the multiple outputs, since each of the output voltages is controlled at the secondary side of the transformer.
FIG. 5 illustrates a conventional switching power supply controlled by a forward magnetic amplifier in which a transformer T2 having a primary side winding N0 and first and second secondary side windings N1 and N2 is provided.
An induced AC voltage at the first winding N1 passes though a rectifying diode D11, a fly-wheel diode D12 and a smoothing circuit comprised of a smoothing coil L11 and a smoothing condenser C11 in order to output a first DC output voltage V01. The output first voltage V01 is fed back to a transistor Q11 serially coupled to the winding N0 at the primary side of the transformer T1 for stabilizing the first output voltage V01 by using a PWM controller 16.
An induced AC voltage at the second winding N2 passes through a rectifying diode D13, a fly-wheel diode D14, a smoothing coil L12 and a smoothing condenser C12 in order to provide a second DC output voltage V02. In order to stabilize the second output voltage V02 independently of the stabilization control for the first output voltage V01, PWM control is performed by using a magnetic amplifier control circuit that includes a saturable inductor LS3.
The magnetic amplifier control circuit includes an operational amplifier OP4. The second output voltage V02 is supplied to a plus (+) input terminal of the operational amplifier OP4 through a variable resistor R22. A reference voltage Vref generated at a Zener diode ZD1 is supplied to a minus (xe2x88x92) input terminal of the operational amplifier OP4 for comparison with the second output voltage V02. By comparing these voltages, the operational amplifier OP4 supplies a control current Im to a conning point between the saturable inductor LS3 and the rectifying diode D13 through a resistor R23 and a diode D15. In FIG. 5, R21 is a current supplying resistance for the diode ZD1.
When the connecting point between the saturable inductor LS3 and the diode D13 has a negative potential, the control current Im flows toward the saturable inductor LS3 from the operational amplifier OP4 through the diode D15. In accordance with the magnitude of the control current Im, the magnetic fluxes of the saturable inductor LS3 are reset. When the magnetic fluxes have reset, the saturable inductor LS3 changes from a saturation state to a non-saturation state.
When the saturable inductor LS3 changes to the non-saturation state, the inductance of the saturable inductor LS3 becomes a large value. Consequently, even when a voltage E is supplied to the saturable inductor LS3 from the winding N2 in a forward direction at a next time after the saturable inductor LS3 has changed to a non-saturation state, the flow of control current Im is delayed by a time xcex94T that corresponds to a reset amount xcex94"PHgr" of the magnetic flux of the saturable inductor LS3. Here, the magnetic flux xcex94"PHgr" is represented as a product of the voltage and the time, i.e., xcex94Txc3x97voltage E. Consequently, the delayed time xcex94T is obtained by the following equation:
xcex94T=xcex94"PHgr"/E. 
Thus, it becomes possible to perform PWM control of the output voltage at the secondary side of the transformer T2 with the changing pulse width of the pulse current flowing through the second winding N2 of the transformer T2 by controlling the amount of the reset of the magnetic flux xcex94"PHgr".
FIG. 6 illustrates a conventional push-pull magnetic amplifier type switching power supply. As illustrated in FIG. 6, a winding N0 is provided at the primary side of a transformer T1 and a plurality of windings N1-N4 are provided at the secondary side of the transformer T1. The winding N0 is coupled to two pairs of transistors F2 and F3, and F1 and F4. By alternately turning ON and OFF the pairs of the transistors F2 and F3, and F1 and F4, it becomes possible to alternately switch the positive and negative current flow through the winding N0 of the transformer T1. C0 is a condenser for preventing saturation of the transformer T1.
Induced AC voltages at the windings N3, N4 are full-wave rectified by a pair of rectifying diodes D21 and D22, and smoothed at a smoothing coil L21 and a smoothing condenser C21. Then a first DC voltage, e.g., 5 V, is outputted at a resistor R31. Further, the induced AC voltages at the windings N1, N2 are full-wave rectified by a pair of rectifying diodes D1 and D2 and smoothed by a smoothing coil L1 and a smoothing condenser C1. Then a second DC voltage, e.g., 10 V, is outputted at a resistor R42. Here, in order to stabilize the output voltage at the resistor R42, a magnetic amplifier control circuit performs PWM control by using saturable inductors LS1 and LS2.
In the magnetic amplifier control circuit, a transistor Q12 is coupled to the resistor R42 through a resistor R41 connected in parallel with the resistor R42. A control current Im outputted from the transistor Q12 is supplied to respective connecting points d1, d2 between each of the saturable inductors LS1, LS2 and each of the rectifying diodes D1, D2, through the respective diodes D3, D4. In FIG. 6, an ON/OFF control circuit for the transistor Q12, such as an operational amplifier, is not illustrated. Thus, stabilization of the output voltage is achieved by resetting the magnetic fluxes of the saturable inductors LS1, LS2 during a period when each of the connecting points d1, d2 has a negative potential, and the pulse current flows in the windings N1 and N2 for the duration of the pulse width.
In the push-pull magnetic amplifier type switching power supply, it is possible to achieve low noise of the switching power supply by using a phase shift full-bridge circuit comprised of the plurality of the transistors F1-F4 on the primary side of the transformer T1.
However, voltages VS1 and VS2 of the respective saturable inductors LS1, LS2 do not only depend upon the magnitude of the control current Im but also depend upon coercive forces Hc1 and Hc2 of the respective saturable inductors LS1, LS2. Consequently, if there are differences between the coercive forces Hc1 and Hc2 of the saturable inductors LS1, LS2, the ON widths of the respective currents that flow to the rectifying diodes D1 and D2 differ from each other even if the same current magnitude flows in both of the saturable inductors LS1, LS2.
Consequently, in a push-pull type switching power supply, the current balance during the push time and the pull time is destroyed, and the condenser CO is DC biased by the current difference. This causes a problem of unbalance of the product of voltage by time for the transformer between the push-time period and the pull-time period, and the transformer T1 is saturated.
Further, in a conventional magnetic amplifier control circuit, it is desirable to insert a common mode choke in order to reduce noise in the output voltages. However, in order to insert the common mode choke in output lines, it is necessary to separate the ground reference from the reference voltage Vref supplied from an outside circuit, Thus, conventionally, it has been difficult to insert the common mode choke in the output lines. Consequently, it has been difficult to reduce noise in the switching power supply.
Furthermore, the conventional switching power supply has another problem of insufficiency of stabilization of the output voltages by only controlling the reset amount of the saturable inductors based on the comparison between the output voltage and the reference voltage.
Therefore, it is desirable to provide a switching power supply that is not subject to the aforementioned problems and disadvantages of a conventional switching power supply.
In accordance with the purpose of the invention as embodied and broadly described, there is provided a switching power supply comprising: a saturable inductor serially coupled to a secondary side winding of a transformer; a smoothing circuit for smoothing an AC output of the secondary side winding of the transformer; an operational amplifier for comparing a DC output voltage from the smoothing circuit and a reference voltage; and a control voltage generating circuit for generating a control voltage of the saturable inductor based on a result of the comparison by the operational amplifier; wherein, a magnetic flux of the saturable inductor is reset in response to the control voltage from the control voltage generating circuit.
Furthermore, there is provided the invention provides a switching power supply, comprising: a saturable inductor serially coupled to secondary side windings of a transformer; a smoothing circuit for smoothing an AC output from the secondary side winding of the transformer; a common mode choke coupled between the smoothing circuit and output terminals of the switching power supply; a first operational amplifier for comparing a DC output voltage from the secondary side winding and a first reference voltage; a current sensor for detecting an output current from the secondary side winding; a second operational amplifier for comparing a voltage corresponding to a detected current by the current sensor and a second reference voltage; a photo-coupler for coupling the first operational amplifier and the second operational amplifier; and a control voltage generating circuit for generating a control voltage based on a result of the comparison by the second operational amplifier; wherein a magnetic flux of the saturable inductor is reset in response to the control voltage from the control voltage generating circuit.
Also in accordance wit the present invention, there is provided a method of operating a switching power supply, comprising: coupling a saturable inductor serially to a secondary side winding of a transformer; smoothing, with a smoothing circuit, an AC output of the secondary side winding of the transformer; comparing, with an operational amplifier, a DC output voltage from the smoothing circuit and a reference voltage; generating a control voltage based on a result of the comparison by the operational amplifier; and resetting a magnetic flux of the saturable inductor in response to the control voltage.
Additionally in accordance with the present invention, there is provided a method of operating a switching power supply, comprising: serially coupling a saturable inductor to a secondary side winding of a transformer; smoothing, with a smoothing circuit, an AC output from the secondary side winding of the transformer; coupling a common mode choke between the smoothing circuit and output terminals of the switching power supply; comparing, with a first operational amplifier, a DC output voltage from the smoothing circuit and a first reference voltage; detecting an output current from the secondary side winding; comparing, with a second operational amplifier, a voltage corresponding to the detected output current and a second reference voltage; coupling, with a photo-coupler, the first operational amplifier and the second operational amplifier; generating a control voltage based on a result of the comparison by the second operational amplifier; and resetting a magnetic flux of the saturable inductor in response to the control voltage.
Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from that description, or may be learned by practicing the present invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.